Objective In the field of next-generation display technologies, organic light-emitting diodes (OLEDs) have attracted significant attention due to their outstanding characteristics such as high brightness, wide color gamut, and flexibility. However, the exciton utilization efficiency of traditional fluorescent OLEDs is relatively low, only reaching 25%. In contrast, thermally activated delayed fluorescence (TADF) materials can theoretically achieve 100% internal quantum efficiency (IQE) through reverse intersystem crossing (RISC). Researchers have designed devices with different sensitization approaches, such as TADF-sensitized fluorescent devices, phosphorescent-sensitized fluorescent devices, and dual-TADF-sensitized devices. The aim is to leverage the high exciton utilization efficiency of sensitizers to overcome the low-efficiency limitation of fluorescent emission. Nevertheless, the long exciton lifetimes of these materials lead to severe efficiency roll-off at high brightness levels. Particularly for red OLEDs, although LI Z et al. achieved a TADF-sensitized DBP-based fluorescent red OLEDs with high color purity in 2021, with its sensitization efficiency reaching 96.4% and the device's external quantum efficiency (EQE) reaching 16.9%, it remains challenging to optimize the device's efficiency and control the roll-off. This study aims to address these issues by constructing a multiple-sensitization system that synergizes TADF, phosphorescent, and fluorescent components, thereby enhancing the exciton utilization and energy transfer processes.

Methods The fabrication of red OLEDs in this research adopted a multilayer architecture fabricated through vacuum evaporation techniques. The emissive layer was carefully designed to consist of the TADF molecule 4CzIPN, the phosphorescent molecule PO-01-TB, and the fluorescent emitter DBP. To optimize device performance, a series of experiments were conducted to adjust the doping concentrations of the sensitizers (4CzIPN and PO-01-TB) and the emitter (DBP). Transient photoluminescence spectroscopy and electroluminescence measurements were employed as key analytical tools. These techniques were used to explore the energy transfer mechanisms, including Förster resonance energy transfer (FRET) and Dexter energy transfer (DET). Additionally, crucial performance metrics such as EQE, current efficiency, and spectral characteristics were comprehensively evaluated to assess the device's performance.

Results and Discussions The multiple-sensitization system demonstrated remarkable performance improvements. It achieved a maximum EQE of 20.97%, representing a significant 25% enhancement compared to single-sensitizer devices (as shown in Fig.3(a) and Tab.2). When the luminance reached 1000 cd·m⁻², the efficiency roll-off decreased from 44.88% in single-sensitizer devices to 37.91% in the multiple-sensitization device (Fig.5(a)). Spectroscopic analysis provided evidence for efficient FRET from 4CzIPN to DBP. Meanwhile, DET played a crucial role in facilitating the capture of triplet excitons by PO-01-TB, effectively minimizing exciton annihilation (Fig.7). In the optimized device, the emission contribution of DBP reached 82.51%, with minimal interference from sensitizer spectra (Fig.7(b)). Transient decay measurements indicated a notable shortening of exciton lifetimes. For instance, at 510 nm, the exciton lifetime of device R6 was 2.11 ns, while that of R1 was 10.64 ns. This reduction verified the enhanced energy transfer kinetics (Fig.9).

Conclusions The 4CzIPN/PO-01-TB/DBP multiple-sensitization system effectively enhanced the performance of red OLEDs, achieving a high EQE of 20.97% and reducing the efficiency roll-off to 37.91%. The synergistic effects of FRET and DET not only improved exciton utilization but also maintained good color purity, with CIE coordinates close to the standard red values. However, there is still room for improvement. Future research should focus on optimizing the energy-level matching between different components and precisely controlling molecular distances. This can further suppress the emission of sensitizers and balance the energy transfer rates, thereby further enhancing the overall performance of red OLEDs.